Polyuretidione adducts of polyisocyanates are intermediates which can be used in the preparation of high performance urethane coatings, paints, and films. These adducts provide reduced volatility and an associated reduced toxicity hazard during use, as compared to monomeric polyisocyanates, such as, for example, toluene diisocyanate. In addition, because of their low viscosity, isocyanato uretidiones can be used as reactive diluents for other highly viscous or solid isocyanate group containing coatings components or as a polyisocyanate component in solvent-free and low solvent coatings formulations.
Processes for preparing these adducts are well known. Examples illustrative of these processes can be found in U.S. Pat. Nos. 4,476,054; 4,912,210; and 4,929,724. Generally, the prior art processes involve adding a soluble catalyst which promotes the isocyanate to uretidione (also known as "dimerization") reaction of the precursor isocyanate, optionally in the presence, but usually in the absence, of a solvent, allowing the reaction to proceed to the desired extent and then stopping the reaction with a suitable quenching agent which destroys the activity of the catalyst. Alternatively, in the cases where relatively volatile catalysts are used, the reaction is stopped by distilling the catalyst along with the residual, unreacted precursor isocyanate from the product dimer.
After the residual, unreacted precursor isocyanate is removed, the resulting material, in the case where the precursor isocyanate is a diisocyanate, is a mixture of oligomers composed of 2, 3, 4, etc. precursor diisocyanate molecules joined by 1, 2, 3, etc. uretidione rings. Usually, this mixture is simply called "dimer".
In the case where the precursor isocyanate is polyisocyanate, the reaction is generally stopped well before all the isocyanate groups have been converted to uretidione groups because, otherwise, the resulting product would be an unusable polymer having a very high (theoretically infinite) molecular weight and viscosity. However, the cost of equipment and energy to remove residual, unreacted precursor isocyanate dictate that the reaction not be stopped too soon. Generally, the reaction is run to more than 10% conversion but less than 50% conversion. The preferred range is between 20 and 35%. The reaction is typically stopped using a quenching agent. The reaction between conventional dimerization catalysts and quenching agents typically results in the formation of an insoluble product which is typically removed by filtration using a filter aid.
Unfortunately, both the quenching agent and the filter aid increase the likelihood of introducing undesirable impurities into the product. Accordingly, catalyst compositions for producing dimers, that do not employ nor require a quenching agent and filter aid(s) during use of the catalyst, would be highly desired by the dimer manufacturing community.
One approach to meeting this need employs a catalyst that is covalently bound to an insoluble organic substrate, as disclosed for example in U.S. Pat. No. 5,015,706. The catalysts disclosed in the '706 patent include so-called "DMAP" and "BMAP" catalysts which fall within the generic classes of catalysts called 4-dialkylaminopyridines and 4-(N-arylalkyl-N-alkyl)amino-pyridines. These classes of catalysts are useful in a variety of reactions, including acylation, urethane formation and uretidione formation. These catalysts have the structure Pyr-NR.sub.1 R.sub.2 (I) where Pyr is a 4-pyridinyl residue and R.sub.1 and R.sub.2 are, independently from one another, C.sub.1 to C.sub.6 alkyl or C.sub.7 to C.sub.12 arylalkyl groups, or, R.sub.1 and R.sub.2, taken together with the attached nitrogen, form a ring which may contain other heteroatoms, such as oxygen, nitrogen or sulfur, to give, for example, pyrrolidine, piperidine or morpholine residues. Common examples of 4-dialkylaminopyridines are 4-dimethylaminopyridine (referred to as "DMAP", structure I where R.sub.1 and R.sub.2 are CH.sub.3) and 4-pyrrolidinylpyridine (structure I where R.sub.1 and R.sub.2, taken together, are (CH.sub.2).sub.4) while 4-(N-arylalkyl-N-alkyl)aminopyridines are exemplified by 4-(N-benzyl-N-methyl)aminopyridine (referred to as "BMAP", structure I where R.sub.1 is CH.sub.2 C.sub.6 H.sub.5 and R.sub.2 is CH.sub.3). A survey of the use of DMAP catalysts, by E. F. V. Scriven, is published in Chem. Soc. Rev., 129 (1983). Another review of DMAP chemistry, by G. Hofle, W. Steglich and H. Vorbruggen can be found in Angew. Chem. Int. Ed. Engl., 17, 569 (1978).
The advantages associated with the use of 4-dialkylaminopyridine and 4-(N-arylalkyl-N-alkyl)aminopyridine catalyst bound to an insoluble matrix, such as the organic polymer matrix disclosed in the above-mentioned '706 patent and the references cited in that patent, have been long recognized. These advantages include the simplified separation of the catalyst from a reaction mixture, the potential of recovering and reusing the catalyst, as well as the ready adaptability of these catalysts for use in static and flow reaction systems.
Unfortunately, it is now recognized that organic polymer substrates have shortcomings during use. These shortcomings are especially evident when the starting materials have two or more reactive sites in each molecule and the resulting products are oligomeric or polymeric, such as polyurethane polyisocyanates or polyuretidione polyisocyanates. These desired products are typically non-volatile liquids or amorphous solids. It is difficult, if not impossible from a practical standpoint, to remove essentially all process contaminants from these types of products. In the former case, where polyurethane polyisocyanates are formed by the catalyzed reaction of, for example, a diisocyanate with, for example, a diol, the resulting product is a mixture of oligomers composed of 2, 3, 4, etc. precursor diisocyanate molecules joined by 1, 2, 3, etc. alcohol residues through urethane bonds. Usually, these types of product mixtures are simply called "isocyanate terminated prepolymers". In the latter case, involving the preparation of polyuretidione polyisocyanates by the catalyzed dimerization of, for example, (cyclo)aliphatic diisocyanates, after removal of the residual, unreacted precursor diisocyanate, the resulting product is a mixture of oligomers composed of 2, 3, 4, etc. precursor diisocyanate molecules joined by 1, 2, 3, etc. uretidione rings. Usually, this mixture is simply called "dimer".
Before 4-dialkylaminopyridine or 4-(N-arylalkyl-N-alkyl)amino-pyridine catalysts bound to an organic polymer can be used to prepare the above-mentioned products, it is necessary to subject the as-produced resin to a rigorous pretreatment to remove low molecular weight substrate oligomers that would otherwise be extracted from the resin and, thereby, contaminate the desired product. Even with such a pretreatment, moderate, but undesirable, levels of color can be formed during the dimerization reaction and in the urethane formation processes. In addition, another, most significant, problem is attributable to the accumulation of product oligomers within the substrate's resin matrix during the reaction. Such fouling continuously reduces the activity of the catalyst resin and thereby limits its useful lifetime. Further, the trapped oligomers can not be easily washed from the substrate. Instead, these entrapped product oligomers represent an undesirable product yield loss and necessitate the use of very rigorous conditions to regenerate the catalyst resin for reuse. Moreover, in some cases the catalysts cannot be regenerated even under rigorous conditions.
While, in principle, the aforementioned problems can be reduced through the use of more highly cross-linked ("macroporous" or "macroreticular") organic polymers, these problems are not entirely eliminated, since residual migration of the reactant(s) into the resin apparently occurs. Further, it is difficult to prepare such polymers having practical levels of catalytic sites available on the surface of the resin bead.
It should be readily apparent, in view of the above discussion, that new catalyst compositions which avoid the above-described problems of catalyst fouling, product yield loss, and product contamination, would be highly desired by the polyisocyante-based coatings manufacturing community. The present invention provides one solution to this problem.